When n parallel bars comprising a code are applied to a document, such as a postal article, the number and the location of the bars indicate the code information. Since these bars are exclusively applied to such documents by means of printing, this method often presents an incomplete print, because one or more bars, which should have been printed in a clearly visible way, are not, or hardly at all, or only partly visible. These errors are often caused by an uneven surface of the documents and by an insufficient supply of printing-ink.